Appliance Design - April 2008 - (Page 15)

COOLING TECHNOLOGIES Fig. 2. Schematic of an acoustic-Stirling cryocooler. Stirling Cycle In a true Stirling cycle, two moving elements combine a volume changing relative motion and a shuttling motion between regions held at different temperatures. The resulting motion can serve as either a cooler or an engine, depending whether the compression (where heat is rejected) happens at the warmer or cooler end; and expansion (where heat is taken from the surroundings) then occurs when the fluid is at the other end. Between ends is a finely-divided porous medium called a regenerator, which is in intimate thermal contact with the fluid at all times, keeping the hot-side heat from leaking through to the cold side, even as the fluid oscillates between the sides. Illustration shows phasing for a refrigerator. Linkage is not shown. < HCFCs has been accelerated. At the same time, price pressures for materials, shipping, and warranty claims are squeezing profits for OEMs. Commodity producers of conventional equipment threaten to stall innovation, and energy-efficiency regulations continue to tighten periodically. All of this has makers of cooling equipment seriously thinking about new approaches. Several alternative refrigerants are subjects of large development projects, but all have significant drawbacks. Carbon dioxide is non-toxic, but it requires extremely high pressures. Ammonia is efficient, but toxic. Light hydrocarbons work well in conventional equipment, but are flammable. No single replacement for conventional refrigerants offers the kind of benefits and acceptability that enable broad acceptance. A way out of this dilemma may be found in a relatively recent development called acoustic-Stirling, which is a variant of thermoacoustic refrigeration. The original thermoacoustic devices used a standing wave, and cleverly managed temperature gradients in the heat transfer process to work as an engine or a heat pump. (See sidebar, Thermoacoustic Engine.) Although simple to build, the intrinsic irreversibility of that arrangement severely limits the attainable efficiency. In a Stirling cycle (see sidebar, Stirling Cycle), a working gas is alternately compressed and expanded while shuttling back-and-forth through a regenerator. (The working gas is usually helium, but hydrogen, nitrogen, and even air have been used.) In the limit, such a device matches Carnot’s efficiency, set by the physical law limiting cooler or heat pump perfor- mance. In practice though, the necessary multiple pistons, seals, and high internal power recirculation have inhibited Stirling machines from achieving widespread commercial application. In an acoustic-Stirling cooler, the core Stirling cycle remains, but most of the machinery is gone, except for a piston on a linear motor, which functions as an extreme loudspeaker. (See Fig. 3.) The linear motor is tuned to resonate at line frequency. That’s because the energy of reciprocating motion is constant at resonance, just like that in constant-speed rotating motion. Just as power is transmitted by a slip-angle in rotation, it is transmitted by a phase angle here. Conversion of AC electric power to oscillating fluid (acoustic) power can be more than 80 percent efficient. All the rest of the pressure and flow control required for a Stirling cycle is achieved by managing the inertia of the fluid itself. That fluid is typically helium for cryogenic versions, but can be nitrogen or even dry air at less-cold conditions where these remain gaseous. With each stroke of the piston, the Fig. 3. Flexure-supported linear motor. applianceDESIGN April 2008 15 www.applianceDESIGN.com http://www.appliancedesign.com

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Appliance Design - April 2008 Contents Editorial Shipments/Forecasts News Watch A Variant of Thermoacoustic Refrigeration Known as Acoustic-Stirling is Expanding Its Range Outside Cryocooling Into More Conventional Applications Such As Refrigeration and Air Conditioning Free-Piston, Stirling Coolers Are Now Showing Up in Mainstream Applications As Costs Come Down and Heat Transport Solutions Are Devised Heat Exchanger Innovations Are Helping To Improve the Overall Efficiency of Vapor Compression Systems Environmental Concerns Have Once Again Ignited a Debate Over Refrigerants A New Glass Ceramic Material for Induction Cooktops Can Be Decorated In a Variety of Glimmering Metallic, 3-D Looks That Can Employ a Number of Shades, Patterns, Styles, Colors, Intensities, and Textures Colorful, Reflective Coatings for Glass Create a Variety of Design Options To Help Designers Establish a Unique Look for Their Products Appliance Designers Faced With Different Motor and Drive Options Need To Understand the Differences and Tradeoffs Before Choosing A New MEMS Valve Offers a Number of Advantages Over Solenoids In Certain Applications, Including the Opportunity To Cut Energy Use The AHR Expo Set Attendance Records and Showcased Thousands of Innovative New Products Classifieds Design Marts Advertiser’s Index Association Report: CEA

Appliance Design - April 2008

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